Position control system

ABSTRACT

A position control system has a speed control loop for generating a torque command signal for a motor from the speed deviation between a speed command and an actual speed. The position control system includes command subdividing means (2) for subdividing the speed deviation (ε(i)) into a predetermined minute amount (ε 2  (i)=α) and a remaining amount (ε 1  (i)=ε(i)-α), integrating meand (3) for integrating the remaining amount (ε 1  (i)), and incompletely integrating means (4) for incompletely integrating the minute amount (ε 2  (i)). The output from the integrating means (3) and the output from the incompletely integrating means (4) are added, and the sum is issued as a torque command signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a position control system for aservomotor or the like, and more particularly to a position controlsystem for precisely controlling fine positioning operation withoutlowering speed gain.

2. Description of the Related Art

Positioning operation in a numerical control apparatus or the likerequires that a movable member be precisely responsive to a finepositioning command. A control system for a servomotor which effectssuch positioning is illustrated in FIG. 5. Denoted at 11 is anarithmetic unit for adding a position command 21 and subtracting aposition feedback signal 22. A converter 12 with a position gain Kconverts the position command issued from the arithmetic unit 11 to aspeed command (u(s)) 23. An arithmetic unit 13 issues a signalindicating the difference between the speed command 23 and a speedfeedback signal 20. An integrator 14 with an integration constant k1integrates the speed command. Designated at 15 is an arithmetic unit forissuing a signal representing the difference between a torque command 18from the integrator 14 and a torque feedback command which is producedby multiplying a speed feedback signal 20 by a proportional gain 19. Acurrent control circuit 16 issues a current dependent on the torquecommand. The reference numeral 17 represents a servomotor. K_(t)indicates a torque constant, J_(m) the inertia of the servomotor, 24 aspeed output from the servomotor, and 25 a position output from theservomotor. The speed output 24 is fed back directly to the arithmeticunit 13 and also fed back to the arithmetic unit 15 after beingmultiplied by a proportional gain k2. The position output 25 of theservomotor is fed back to the arithmetic unit 11.

Operation of the position control system thus constructed is shown inFIG. 6. The graph of FIG. 6 has a horizontal axis indicating a movementcommand in a unit of 1 μm and a vertical axis representing actualmovement of a mechanical movable member in a unit of 1 μm. Ideally, amechanical movable member would move precisely 1 μm each time a commandfor 1 μm is applied, as indicated by the straight line M1.

Actually, however, as indicated by the polygonal line M2, a mechanicalmovable member moves 0.2 μm at a time in response to a command for 1 μmand moves 1.8 μm at a time in response to the next command for 1 μm, forexample, and hence does not move in exact response to applied commands.Thus, the mechanical movement is polygonal due to the so-calledstick/slip phenomenon. This phenomenon is responsible for a reduction inthe accuracy of actual operation of the mechanical movable member andfor a poor finishing surface.

The causes of such undesirable conditions will be analyzed below. FIG. 7shows the torque command illustrated in FIG. 5. The graph of FIG. 7 hasa horizontal axis indicative of time (t) and a vertical axis of torque(T). When a position command 21 for 1 μm is applied, the torque command19 issued from the integrator 14 of FIG. 5 increases along a straightline C1 as shown in FIG. 7. When the torque exceeds a static frictiontorque C3, the servomotor 17 starts rotating. There is a considerableperiod of time before a position output is actually fed back. Duringthat period of time, the torque command increases. When the servomotorstarts to rotate, the servomotor moves beyond the command value sincethe dynamic friction torque is much smaller than the static frictiontorque. As a result, the amount of movement when a next command for 1 μmis applied becomes smaller than 1 μm.

To solve the above problem, the integration constant k1 of theintegrator 14 shown in FIG. 5 may be made small to cause the torque toincrease along a curve C2 as shown in FIG. 7, so that the torqueincreases more gradually. However, such a solution is still problematicin that when the position command is large, the response of the entiresystem is slow.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a position controlsystem for precisely controlling fine positioning operation withoutlowering speed gain.

In order to eliminate the aforesaid problems, there is provided inaccordance with the present invention a position control system having aspeed cntrol loop for generating a torque command signal for a motorfrom the speed deviation between a speed command and an actual speed,the position control system comprising: command subdividing means forsubdividing the speed deviation into a minute amount and a remainingamount which are defined in advance; integration means for integratingthe remaining amount; incompletely integrating means for incompletelyintegrating the minute amount; and arithmetic means for adding an outputfrom the integrating means and an output from the incompletelyintegrating means, and issuing the sum as a torque command signal.

Since the speed deviation is divided into a minute amount and aremaining amount, when the command value is minute, the gradient of thetorque curve is reduced by the incompletely integrating means. When thecommand value is large, there is a remaining amount, and the torqueincreases along the conventional torque curve. Therefore, the torquecurve varies according to the command value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of the present invention;

FIG. 2 is a block diagram showing a conventional torque commandintegrator as a discrete system;

FIG. 3 is a flowchart of operation of a command dividing means accordingto the embodiment of the invention;

FIG. 4 is a diagram showing a fine positioning command and an actualmovement of a mechanical movable member according to the embodiment ofthe invention;

FIG. 5 is a diagram showing a conventional servomotor position controlsystem as an analog system;

FIG. 6 is a diagram illustrating a fine positioning command and anactual movement of a mechanical movable member according to theconventional system; and

FIG. 7 is a diagram showing the relationship between a torque commandincrease and a static friction torque.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will hereinafter be described inspecific detail with reference to the drawings.

FIG. 1 is a block diagram of an embodiment according to the presentinvention. Since digital processing is carried out in this embodiment,the block diagram is shown as a discrete system. FIG. 1 corresponds toand replaces the integrator 14 shown in FIG. 5 (which is indicated by Ain FIG. 5). The integrator 14 of FIG. 5 may be expressed as a discretesystem as shown in FIG. 2.

Denoted at 1 in FIG. 1 is an arithmetic unit for calculating and issuingthe difference between a speed command u(i) from a converter (not shown)and a speed feedback signal Y(i) from a servomotor.

A command subdividing means 2 serves to subdivide a difference outputsignal ε(i) from the arithmetic unit 1 into:

a minute amount ε₂ (i)=α

a remaining amount ε₁ (i)=ε(i)-α

Generally, α is selected to be of a value in the vicinity of a speeddeviation corresponding to a minimum detected unit or a minimum commandunit (1 μm) of a position feedback system, and is experimentallydetermined through observation of a minute movement amount command andan actual movement amount as shown in FIG. 6. An output signal from thecommand subdividing means 2 is applied to an integrating means 3 and anincompletely integrating means 4.

The remaining amount ε₁ (i) is generated when the command value islarge, and is subjected to conventional integration. The integratingmeans 3 comprises an arithmetic unit 5, a Z function Z⁻¹, and a gain k1,and has the same function as that of the integrator 14 shown in FIG. 5.Since the integrating means 3 actually processes the signal as adiscrete system, it effects the same processing as that of the block Ain FIG. 2. Therefore, when the command value is large, the processing isthe same as the conventional process. The gain k1 is of the same valueat that shown in FIG. 5.

The minute amount ε₂ (i) is applied to the incompletely integratingmeans 4. The incompletely integrating means 4 is composed of anarithmetic unit 6, a Z function Z⁻¹, a gain k2, and a feedback gain k3for the Z function Z⁻¹. The output from the integrating means 3 is heldeven if a position feed signal is fed back to reduce a speed deviationsignal to zero, whereas the output of the incompletely integrating means4 is exponentially reduced when the speed deviation signal is reduced tozero. The output signal from the integrating means 3 and the outputsignal from the incompletely integrating means 4 are added by anarithmetic unit 7, which issues the sum as a torque command signal.

As described above, if the position command signal is minute, the speeddeviation signal is subdivided by the command subdividing means 2 andprocessed by the incompletely integrating means 4. Therefore, when thespeed deviation signal is fed back to eliminate or reduce the speeddeviation, the torque command signal is reduced, and excessive movementof the servomotor is suppressed, so that the accuracy of actual movementwith respect to the minute command is increased. If the position commandis large, the remaining amount of the speed deviation is large andprocessed by the conventional integrating means 3 without lowering aspeed gain.

Consequently, a system is established wherein the speed gain is smallfor a minute position command and the speed gain remains the same asconventional for normal position commands.

Processing operation of the command subdividing means 2 will bedescribed below. FIG. 3 shows a flowchart of processing operation of thecommand subdividing means 2. Step S1 determines whether or not theabsolute value |ε(i)| of the speed deviation ε(i) is equal to or greaterthan a minute amount α. If it is equal to or greater than the minuteamount α, then control goes to a step 3. If it is smaller than theminute amount α, i.e., if the position command is of a minute amount,then control goes to a step 2.

In step S2, the speed deviation input ε(i) is issued as a minute amountε₂ (i). In step S3, the speed deviation is larger than a minute amount,and it is first determined whether the speed deviation is positive ornegative, because the speed deviation is either positive or negativedependent on whether the position command is positive or negative. Ifthe speed deviation ε(i) is positive, then control proceeds to a step 4(S4), and if the speed deviation ε(i) is negative, then control goes toa step 5 (S5).

In step S4, since the speed deviation ε(i) is positive, it is dividedinto a remaining amount and a minute amount according to the followingequations:

a remaining amount ε₁ (i)=ε(i)-α

a minute amount ε₂ (i)=α

In step S4, since the speed deviation ε(i) is negative, it is subdividedinto a remaining amount and a minute amount according to the followingequations:

a remaining amount ε₁ (i)=ε(i)+α

a minute amount ε₂ (i)=-α

The speed deviation ε(i) can thus be subdivided into a minute amount anda remaining amount by the above steps. The hardware of the presentembodiment can easily be implemented by a known microcomputer.

Results of an experiment conducted on the present embodiment are shownin FIG. 4, which is similar to FIG. 6. The straight line M1 representsideal movement of a mechanical movable member, and the polygonal line M3indicates movement of the mechanical movable member controlled by theembodiment of the present invention. Comparison between the polygonalline M3 and the polygonal line M2 shown in FIG. 6 indicates thatmovement of the mechanical movable member according to the presentinvention is improved.

Although in the above embodiment the speed deviation is alwayssubdivided into a minute amount and a remaining amount, even when it islarge, it is possible to process the speed deviation only as a remainingamount when it is large.

With the present invention, as described above, the speed deviation issubdivided into a minute amount and a remaining amount as defined inadvance, and the minute amount is incompletely integrated whereas theremaining amount is integrated. The amounts thus integrated are added,and the sum is used as a torque command signal. Therefore, positioningcontrol can be effected without reducing a speed gain in response to alarge position command, and precise movement can be achieved in responseto a minute position command.

We claim:
 1. A position control system having a speed control loop forgenerating a torque command signal for a motor from a speed deviationbetween a speed command and an actual speed of the motor, said positioncontrol system comprising:command subdividing means for dividing thespeed deviation into a minute amount and a remaining amount; integratingmeans for integrating the remaining amount to produce a first output;incompletely integrating means for incompletely integrating the minuteamount to produce a second output; and arithmetic means for adding thefirst output from said integrating means and the second output from saidincompletely integrating means, to produce a sum issued as the torquecommand signal.
 2. A position control system according to claim 1,wherein said integrating means and said incompletely integrating meansare arranged to process each of the remaining and the minute amounts,respectively, with a discrete system.
 3. A position control systemaccording to claim 1, wherein said command subdividing meanscomprises:first determining means for determining if the absolute valueof the speed deviation is greater than or equal to the minute amount;and second determining means for determining whether the value of thespeed deviation is greater than zero.
 4. An integrator circuit in aposition control system for generating a torque command signal, theposition control system including a first arithmetic unit for generatingan adjusted position signal upon receiving a position command signal anda position feedback signal from a motor, a converter to convert theadjusted position signal into a speed command signal and a secondarithmetic unit for generating a speed deviation signal upon receivingthe speed command signal and a speed feedback signal from the motor saidintegrator circuit comprising:command subdividing means, operativelyconnected to the second arithmetic unit, for subdividing the speeddeviation signal into a predetermined minute amount and a remainingamount; integrating means, operatively connected to said commanddividing means and having a first integration constant, for integratingthe remaining amount to produce an integrated remaining amount;incompletely integrating means, operatively connected to said commanddividing means and having a second integration constant, forincompletely integrating the predetermined minute amount to produce anintegrated minute amount; and a third arithmetic unit, operativelyconnected to said integrating means and said incompletely integratingmeans, for generating the torque command signal by summing theintegrated remaining and minute amounts.
 5. A position control systemaccording to claim 4, wherein the second integration constant is smallerthan the first integration constant.
 6. A position control systemaccording to claim 5, wherein said command subdividing meanscomprises:first determining means for determining if the absolute valueof the speed deviation signal is at least as large as the predeterminedminute amount; first assigning means for assigning the value of thespeed deviation signal to the predetermined minute amount if said firstdetermining means determines the absolute value of the speed deviationsignal to be less than the predetermined minute amount; seconddetermining means for determining whether the value of the speeddeviation signal is greater than zero after said first determining meansdetermines the absolute value of the speed deviation signal to be atleast as large as the predetermined minute amount; and second assigningmeans for assigning a polarity to the predetermined minute amount independence upon the determining of said second determining means.
 7. Amethod for generating a torque command signal for a motor in a positioncontrol system upon receiving a speed deviation signal resulting fromthe difference between a speed command signal and a speed feedbacksignal from the motor, said method comprising:(a) subdividing the speeddeviation signal into a minute amount and a remaining amount; (b)integrating the remaining amount to produce a first result; (c)incompletely integrating the minute amount to produce a second result;and (d) producing the torque command signal by adding the first resultand the second result together.
 8. A method according to claim 7,wherein said incompletely integrating of step (c) is performed with thesecond integration constant smaller than the first integration constant.9. A method according to claim 8, wherein said subdividing in step (a)comprises:(ai) determining if the absolute value of the speed deviationsignal is less than the minute amount; and (aii) setting the minuteamount equal to the speed deviation signal, if said determining in step(ai) determines the absolute value of the speed deviation signal to beless than the minute amount.
 10. A method according to claim 8, whereinsaid subdividing in step (a) comprises:(ai) determining if the absolutevalue of the speed deviation signal is at least as large as the minuteamount; (aii) determining if the value of the speed deviation signal isless than zero; and (aii) setting the sign of the minute amount negativeif said determining in both steps (ai) and (aii) is affirmative.